The present invention relates to radio frequency amplifiers and particularly relates to a variable gain driver amplifier.
Wireless communication systems enjoy significant popularity, finding widespread use in both developed and developing regions. Indeed, the very popularity of wireless communication systems spurs their development and advancement, driving system designers and service providers to devise ways of supporting more users within a finite radio frequency spectrum. Existing and pending wireless communications standards typically rely on frequency, code, or time-division multiplexing techniques that allow multiple users of mobile terminals to share the same frequencies within a given service area. Commonly, such access schemes benefit from each mobile terminal controlling its transmitted signal power to help minimize its interference with other active devices in a given wireless service area.
This power control approach presents mobile terminal designers with significant challenges. For example, power control techniques typically require transmit signal amplifier circuits that provide a range (often continuous) of transmit signal gain. This allows a controlling device (e.g., a cellular telephone) to transmit with a desired signal power based on adjusting the gain of one or more such transmit signal amplifier circuits. For example, as the mobile terminal moves closer to a supporting base station, the wireless communications systems may instruct the mobile terminal, via control signaling, to reduce its transmit power. Essentially, in such wireless communications systems, active mobile terminals are controlled such that they transmit with the minimum necessary signal power at all times.
High fidelity requirements impose further challenges on designers of wireless communications transmitters. Many wireless communications standards impose strict adjacent power limitationsxe2x80x94a measure of unwanted signal power appearing in radio channels adjacent to the selected transmit channel. Digital modulation schemes, such as those used in Global System for Mobile communications (GSM) or Personal Communications System (PCS) systems, typically require phase or frequency modulation in combination with amplitude modulation. The need for envelope modulation (amplitude) imposes a requirement for linear amplification of the transmit signal. As noted, this linear amplification must usually support variable gain, so that the linear, radio frequency signal may be gain adjusted to comply with transmit signal output power control requirements.
Thus, modern wireless communication devices typically must meet the dual, sometimes contradictory, requirements of providing flexible transmit signal output power control, while still maintaining acceptable amplification linearity. A number of approaches exist for meeting these design challenges, and include transistor-based amplifier gain stages using differing topologies and differing gain control techniques.
One approach to a transistor amplifier gain stage comprises providing a transistor amplifier having a gain path and a shunt path, with the shunt path having no signal gain. In operation, a current steering mechanism splits current between the gain and shunt paths to provide a desired amount of gain. A xe2x80x9cGilbertxe2x80x9d cell represents such a configuration for a differential amplifier.
Typically, before a power amplifier is used, a driver amplifier with two amplifier gain stages is used on the RF input signal. This arrangement requires voltage drops across at least two transistors to keep both transistors operating properly. As a result, the collector of the transistor in the second stage may have a limited voltage swing, which in turn limits the amplifier""s ability to generate high, linear output power levels.
Accordingly, there remains a need for a driver amplifier that exhibits increased headroom in the amplifier to provide high, linear output power levels.
The present invention allows an input amplifier stage to couple its output power to a second gain stage through a transformer. The transformer coupling allows for the removal of a diode voltage drop at the second gain stage, thereby allowing the collector of the transistor in the second stage to have a larger voltage swing.
In particular, an RF input may be provided at the base of a first transistor and may be amplified at the collector of the first transistor. This amplified RF signal is then coupled across the transformer such that it provides an input to an emitter on a second transistor. The second transistor has a variable bias based on the amount of gain desired. The voltage level at the collector of the second transistor is free to swing between greater values, because the minimum voltage level at the emitter is not required to be sufficient to bias the first transmitter correctly.
In one embodiment, the second transistor comprises part of a second stage in which a current steering arrangement insures a well-defined minimum gain setting and good amplifier linearity over the gain control range. The second stage is configured with parallel output current paths: a gain path and a shunt path. The gain path comprises an impedance element that develops an output signal voltage in response to the amplifier""s time varying output current, while the shunt path is configured to be low-impedance. A current steering circuit determines how the amplifier output current splits between the gain and shunt paths, thereby controlling amplifier gain.